1. Field of the Invention
This invention relates to a nitride semiconductor light emitting element which can function with a lower driving voltage than that of a conventional light emitting element without losing essential functions of the light emitting element.
2. Description of the Related Art
Nowadays, nitride semiconductors such as gallium nitride (GaN), nitriding indium gallium (InGaN), gallium nitride aluminum (AlGaN) come into the limelight as a material for a green-blue-near ultraviolet region light emitting element (LED).
FIG. 6 is a diagram showing a configuration example of a conventional nitride semiconductor light emitting element.
A light emitting element 200 comprises a sapphire substrate 201, a low-temperature grown GaN buffer layer 202, an undoped GaN layer 203, a n-GaN layer 204, a pair multiquantum well (MQW) 205 including pairs of InaGa1-aN (well layer)/GaN (barrier layer), a p-Al0.1Ga0.9N layer 206, and a p-GaN layer 207 sequentially grown on the sapphire substrate 201. A surface of the p-GaN layer 207 to a part of the n-GaN layer 204 are etched by a reactive ion etching (RIE) apparatus, and a transparent conductive film 208 (Ni with a thickness of 2 nm/Au with a thickness of 4 nm) is deposited on the surface of the p-GaN layer 207 which is not etched by the RIE. Further, a p-electrode 209 is formed on a region of the p-GaN layer 207 which partially overlaps the transparent conductive film 208, and a n-electrode 210 is formed on the surface of the n-GaN layer 204 which is etched by RIE.
For example, for the case of a blue light emitting element with a peak wavelength of 460 nm, the pair multiquantum well 205 comprising pairs of the InaGa1-aN well layer/GaN barrier layer has an In composition ratio a is 0.15, and is formed at a growth temperature of 780° C. On the other hand, for the case of a green light emitting element with a peak wavelength of 525 nm, the pair multiquantum well 205 comprising pairs of the InaGa1-aN well layer/GaN barrier layer has an In composition ratio a of 0.25 and is formed at a growth temperature of 730° C.
In the conventional blue or green nitride semiconductor light emitting element, it was necessary for applying a voltage not less than 3V to flow an electric current with a current density of 20 A/cm2. This voltage value is the value which is remarkably large in comparison with energy of the emission light (2.7 eV when the peak wavelength of the emission light is 525 nm for the case of the green light emitting element, and 2.36 eV when the peak wavelength of the emission light is assumed 460 nm for the case of the blue light emitting element). Such a difference in energy generates heat inside the semiconductor and raises a temperature in operation of the nitride semiconductor light emitting element, thereby causing fall of output power, fall of lifetime of the light emitting element, degradation of a sealing resin or the like.
The reason of the above effects is assumed as follows. In the conventional nitride semiconductor light emitting element, a GaN layer as shown in FIG. 6 or InGaN including a little of In (In composition ratio is not greater than 3%) is used as a barrier layer of the multiquantum well, so that a bandgap of the barrier layer increases more than 3V, and such a large bandgap disturbs dislocation of carrier in the barrier layer. In accordance with the above, it is assumed that it is effective to reduce the bandgap of the barrier layer, for example, by using the InGaN layer with the In composition ratio of not less than 4% as a barrier layer, so as to reduce a driving voltage of the nitride semiconductor light emitting element. Such a technique is disclosed by Japanese Patent No. 3135041 (JP-B-3135041, Embodiment 2, FIG. 1) and Japanese Patent Laid-Open No. 2002-76521 (JP-A-2002-76521, FIG. 1).
However, in the light emitting element disclosed by Japanese Patent No.3135041, since an In composition ratio of the barrier layer in the multiquantum well is 20%, lattice mismatch between a n-type GaN layer (a backing layer) and an active layer is increased, so that a new and further lattice defect is introduced into the active layer. As a result, the aforementioned light emitting element may not function as a light emitting element.
In addition, in the light emitting element disclosed by Japanese Patent Laid-Open No.2002-76521, since a light guide layer having a composition same as a barrier layer comprising In0.05Ga0.95N is grown to have a thickness of 200 nm, crystal defect based on lattice mismatch is also introduced. As a result, there is a risk that a light emitting function of the light emitting element may be lost.